The invention relates to a process and a circuit for adapting a post integration in a switched pulse repetition frequency (PRF) Doppler radar system.
In a low frequency radar, the relatively low pulse repetition frequency implies, for a Doppler filter used at low altitude, a pattern (such as shown in FIG. 1) in which the ratio between the rejection zone and the passing zone is high (40%). Since a large number of aerial targets whose ambiguous speed corresponds to the rejection zone of the filter are not detected, the recurrence frequency FR is varied during the illumination time of the target which shifts the ambiguous frequencies of the targets, without disturbing the echoes whose frequency is less than FR/2.
All the aerial targets are then detected during a fraction of the illumination time corresponding to the recurrence frequencies such that their ambiguous frequencies fall into the passing zone of the filter. It follows that the echo is chopped, therefore the conventional post integration is not adapted to this type of echo.
In fact, according to the known technique, the post integration is based:
either on a total illumination time, which causes losses when the target only appears at the output of the Doppler filter for a small fraction of the illumination time,
or on an average or small fraction of this illumination time, which causes a loss when the echo is present for a large fraction of this illumination time.
A considerable loss of detection accuracy is therefore observed as well as a mediocre quality picture on the indicator (the echo appears as dots on the screen).
The invention aims at overcoming the above mentioned disadvantages by adapting a type of post integration to the useful signal actually received after the Doppler filter.